Question

我目前正在学习SQL，我无法理解为什么这两个查询会返回不同数量的行（第一个返回53，而第二个返回69）。

SELECT d.department_name "dept_name",
   j.job_title "job_title",
   e.manager_id "manager_id",
   MAX(e.salary) "max_salary",
   SUM(e.salary) "sum_salary"
FROM employees e, jobs j, departments d
WHERE j.job_id = e.job_id AND
      d.department_id(+) = e.department_id
GROUP BY GROUPING SETS( (d.department_name, j.job_title), (j.job_title,e.manager_id), ());

第二个：

SELECT d.department_name AS "dept_name",
j.job_title AS "job_title",
  e.manager_id AS "manager_id",
    MAX(e.salary) AS "max_salary",
      SUM(e.salary) AS "sum_salary"
FROM employees e
INNER JOIN jobs j ON j.job_id = e.job_id
RIGHT OUTER JOIN departments d ON d.department_id = e.department_id
GROUP BY GROUPING SETS( (d.department_name, j.job_title), (j.job_title,e.manager_id),  ());

感谢您的帮助！

Answer 1

在旧式连接语法

FROM employees e,  departments d
WHERE d.department_id(+) = e.department_id

是左连接，等于：

FROM employees e
LEFT JOIN departments d ON d.department_id = e.department_id

对于这些查询，两个工作完全相同，您还应该在第二个查询中将RIGHT JOIN更改为LEFT JOIN，或者将d.department_id（+）= e.department_id更改为d.department_id = e.department_id（+）in第一个。

Answer 2

您的查询不同，因为在第一个外部联接表部门，而在第二个外部联接作业和员工。（这就是为什么我不喜欢右外连接。我觉得它们不太可读。）你带员工，加入工作然后通过RIGHT OUTER JOIN你说：并且无论如何都给我所有的部门，所以给我为工作和员工提供额外的（外部联合记录）。第二个陈述等于：

SELECT d.department_name AS "dept_name",
j.job_title AS "job_title",
  e.manager_id AS "manager_id",
    MAX(e.salary) AS "max_salary",
      SUM(e.salary) AS "sum_salary"
FROM departments d 
LEFT OUTER JOIN employees e ON e.department_id = d.department_id
LEFT OUTER JOIN jobs j ON j.job_id = e.job_id
GROUP BY GROUPING SETS( (d.department_name, j.job_title), (j.job_title,e.manager_id),  ());

在旧的Oracle语法中，您会写：

SELECT d.department_name "dept_name",
   j.job_title "job_title",
   e.manager_id "manager_id",
   MAX(e.salary) "max_salary",
   SUM(e.salary) "sum_salary"
FROM employees e, jobs j, departments d
WHERE e.department_id(+) = d.department_id
AND j.job_id(+) = e.job_id;

新旧SQL JOINS之间的区别

2 个答案: